Stable suspensions of metal oxide nanoparticles are important for a variety of applications and processes. Basically, the formation of stable suspensions can be achieved by two methods (alone or together): electrical charging of the surface to cause coulombic repulsion between the particles and steric effects that are usually achieved by the adsorption of organic materials on the nanoparticles surface. Depending on the material and application, one should choose the preferred method with the understanding that the adsorption of materials on the particles surface induces impurities to the system. For some applications such as optic, electronic, optoelectronic, solar energy conversion and others, it is important to process at low temperatures (around 500° C. for inorganic devices and less than 150° C. for plastic based systems) and minimize the contamination of final products with non-volatile impurities.
One of the possible applications of stable suspensions of charged metal oxide particles is electrophoretic deposition (EPD). EPD is a useful way for the preparation of thick binder-free particulate films on conductive substrates. In the EPD method, the charged particles in suspension are moved to the substrate by the applied voltage, and then are deposited on the substrate. In this case, particle charging provides two effects: coulombic repulsion for the formation of a stable suspension and charging that allows particles movement to the opposite charged electrode. No deposition of the particles by EPD occurs if they are not charged.
A wide variety of additives can be used to charge particles and thus produce well-dispersed suspensions (Sarkar and Nicholson, 1996; Zhitomirsky, 2002; Koura et al., 1995; Zhitomirsky, 1998; Special Issue on “Electrophoretic Deposition: Fundamentals and Applications”, 2004). Most of the dispersing and charging additives are oligomeric or polymeric ionic or polar organic compounds, such as polyethylenimine, phosphate ester, poly(dimethyl-diallylammonium chloride), trioxadecanoic acid and acrylate-acrylamide copolymers. These dispersing and charging additives are usually applied for a dense ceramics producing, and removed during sintering at high temperatures (1200-1700° C.).
A general system composed of iodine, acetone and water as additives for metal oxide particles charging in suspensions with polar organic liquid is also known (Zhitomirsky, 2002; Koura et al., 1995; Zhitomirsky, 1998). The advantage of this composition derives from the ability to remove all of its components from produced metal oxide films at quite low temperature.